Isoflavones contained in soybeans are widely known to have the effect (the anti-estrogenic effect) of preventing breast cancer, prostate cancer, and the like; as well as the effect (the estrogen-like effect) of ameliorating menopause symptoms, postmenopausal diseases such as osteoporosis, hyperlipidemia, hypertension, and the like.
In addition to the direct clinical effects of soybean isoflavones, recent reports suggest that equol, which is an active metabolite of the soybean isoflavones, is the key to effectiveness in clinical applications, instead of the soybean isoflavones. Specifically, various reports state that equol is effective against breast cancer, prostate cancer, menopause symptoms, and postmenopausal osteoporosis.
Further, it has been reported that because equol is produced by intestinal bacteria, the amount of production is different depending on intestinal flora, and thus varies among different individuals. Accordingly, it is undeniable that there are some individuals who cannot experience a desired anti-estrogenic effect and estrogen-like effect, even if they ingest soybean processed products.
Some time later, lactic acid bacteria that efficiently produce equol were isolated and identified (Patent Literature 1). An efficient intake of equol into the body is expected to be achieved by orally ingesting a lactic acid bacteria preparation, and allowing the lactic acid bacteria to reach or colonize in the intestine. At this time, it is possible to confirm whether the lactic acid bacteria have reached or colonized in the intestine by measuring equol in, for example, urine, blood, feces, etc.
The following methodes are known as methods to measure equol: a method using a liquid chromatograph-mass spectrometer or a gas chromatograph-mass spectrometer in the case of blood samples, and a method using instrumental analysis such as high-performance liquid chromatography (HPLC), etc. in the case of urine samples. However, a simpler measurement method is demanded.
Further, as for the methods for measuring equol by immunoassay, Labmaster TR-FIA Research Reagents for the Measurement of Equol, which uses time-resolved fluoroimmunoassay, is commercially available (NPL 1).
Meanwhile, the presence of two types of diastereoisomers of equol, i.e., S-equol and R-equol, is known. Between them, S-equol is the only metabolite produced from daidzein (a precursor of equol) by intestinal bacteria. Further, S-equol binds with an estrogen-receptor much more strongly (NPL 2).
Specifically, S-equol is the equol that indicates its presence in vivo, and that is contained in blood samples and urine samples; it is thus useful to specifically measure S-equol.